Some Design Patterns In Ruby

Some Design Patterns In Ruby

Just because you have duck-typing doesn't mean you can ignore common OO idioms!

https://github.com/abramhindle/ruby-design-patterns http://softwareprocess.es [email protected]

(c) 2012 Abram Hindle and Ken Wong CC-BY-SA 3.0

Singleton Pattern ●

Restrict instatiation of a class to 1 object.

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Allow only 1 instance of a class.

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Like a global, but hopefully less mutable.

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Can be lazily instantiated

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Much maligned.

Singleton Pattern ●

Necessary UML MySingleton Creates and Limits access

Instance: MySingleton

getInstance(): MySingleton

Square boxes are for suckers

Singleton Pattern ●

In Ruby you generally limit access via a gentleman's agreement we won't peak the guts. –

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I think we should avoid overcomplicating things and stick to this agreement. # http://apidock.com/ruby/Module/private_class_method # http://dalibornasevic.com/posts/9-ruby-singleton-pattern-again class MySingleton # attempt to limit access to the constructor private_class_method :new # class variable instance @@instance = nil def self.instance() if (@@instance.nil?()) # self. and MySingleton tend not to work @@instance = self.new() end return @@instance end end

Singleton ●

There are many more ways to make a singleton

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The singleton module is pretty useful require 'singleton' # http://dalibornasevic.com/posts/9-ruby-singleton-pattern-again class IncSingleton include Singleton end

Singleton Resources ●

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Ruby Singleton Pattern Again http://dalibornasevic.com/posts/9-ruby-singleton-pattern-again Dalibor Nasevic Singleton Pattern http://c2.com/cgi/wiki?SingletonPattern Singleton Pattern http://en.wikipedia.org/wiki/Singleton_pattern Method private_class_method http://apidock.com/ruby/Module/private_class_method

Composite Pattern ●

Design intent: –

to compose individual objects to build up a tree structure ●

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The individual objects and the composed objects are treated uniformly –

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e.g., a folder can contain files and other folders

e.g., files and folders both have a name or a size

Each object is responsible for answering its own query.

“Recursive (De)composition” a Folder

a File

a Folder

a Folder

a File

a File

a File

a File

a Folder

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Composite Example Structure NamedSizedItem

0..*

+getSize(): int

File

Folder

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Composite Example Structure Component

0..*

+action(): value

Leaf

Composite

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Composite Code class NamedSizedItem @name = nil @size = 0 def initialize(name) @name = name end def name() return @name end def size() return @size end end class NamedFile < NamedSizedItem def initialize(name) @name = name @size = 1 end end

class NamedFolder < NamedSizedItem @files = [] def initialize(name) @name = name @files = [] end def addFile(file) @files